This invention relates generally to television apparatus, and more particularly to a method and apparatus for digitally encoding and processing an NTSC color television signal.
In digital television systems, it is necessary to reduce the bit rate of the digital television signals, usually pulse code modulated (PCM), whenever a transmission channel or a digital store is limited in capacity. One way of doing this is to lower the PCM encoding frequency, F.sub.s ; however, the Nyquist sampling limit is soon reached, and further reduction in sampling frequency results in beat distortions due to "alias components" when the lower sidebands of F.sub.s overlap the baseband video frequencies; since the baseband video bandwidth, F.sub.v, for the NTSC system of television is 4.2 MHz, the Nyquist sampling limit is reached when F.sub.s 2F.sub.v, i.e., F.sub.s =8.4 MHz.
It is known from applicant's U.S. Pat. No. 4,065,784 that NTSC color television signals can be digitally encoded at sub-Nyquist rates by placing the alias components into those parts of the spectrum not normally occupied by the luminance or chrominance components of the video signal. In the patented system, F.sub.s is exactly 2F.sub.sc 1/4F.sub.h or 2F.sub.sc -1/4F.sub.h, where F.sub.sc is the NTSC color subcarrier frequency and F.sub.h is the line-scan frequency. Most of the alias signals in the thus-encoded signal are removed from the baseband video by comb filtering between (F.sub.s -F.sub.v) and F.sub.v.
It has recently been proposed that the NTSC color television signal be encoded at a sampling rate four times the color subcarrier frequency (i.e., 4F.sub.sc) in order to maintain picture quality and ease digital processing. Encoding at this sampling frequency results in a bit rate of 114 Mbs, which may be excessive for certain applications. Although sub-Nyquist encoding is a way to reduce the bit rate, the quarter line frequency offset described in the aforementioned patent makes it difficult and expensive to obtain the correct samples from a 4F.sub.sc encoded PCM television signal.
It is the primary object of the present invention to provide a method and apparatus for obtaining a sub-Nyquist sampled signal directly from 4F.sub.sc samples without the need for interpolation, from which the 4F.sub.sc samples can again be readily reconstructed, using comb filters to remove alias components.
Another object of the invention is to provide a method and apparatus for obtaining from an analog NTSC color television signal a sub-Nyquist encoded PCM signal from which 4F.sub.sc samples can be readily reconstructed.